In recent years, overnight letter service has grown at a very high rate. In order to provide prompt service and at the same time be able to bill the user, various types of envelopes have been utilized so that the airbill is readily viewable. In one form of envelope, a transparent sheet is adhesively secured to a flexible envelope on three sides to form a pouch. After the airbill has been inserted into the pouch, the remaining side is adhesively secured to the envelope. In another form, an envelope is formed from a sheet of cardboard with side panels on a first panel being secured to the inner surface of a second panel. Before the side panels are secured, a flexible transparent sheet is superposed over the outer surface of the one panel, surfaces of the side panel and a portion of the outer surface of the second panel. Therefore, a portion of the flexible transparent sheet is adhesively secured to the inner surface of the one sheet and the side panels. An overlapping slit is supplied with the transparent flexible sheet so that the slit may be parted and the airbill inserted therein. While these envelopes function to perform a service, it was felt that improvements could be made in the envelopes.